This invention relates to an improved surface following device using pneumatic pressure to maintain a probe a constant distance from the surface of a workpiece. This invention is particularly useful in maintaining an eddy current coil a specified distance from a metallic workpiece whose surface configuration is irregular.
Eddy current probes must be kept a constant distance from the surface of the workpiece under inspection if the measurements are to be a useful indication of the quality of that surface. This is a difficult task especially when the surface of the workpiece, such as a turbine rotor plate, is irregular.
Prior art eddy current probes currently used for non-contact inspections of engine parts use compressed air as a cushion to force the tip of the probe away from the part surface. The lift-off distance is sensitive to probe tip area and configuration tolerances. Variation in area changes the lift-off distance, resulting in variability among probes of the same type. Small probe tip areas, needed for inspecting parts with small radii, develop a reduced air cushion that results in low (or no) lift-off; thus, the probe tip touches the part more frequently during inspection, and wears out more quickly. Clearly, there is a need for a probe to follow the part surface with a constant lift-off distance, independent of surface area. Some attempts at using a closed-loop control system were tried, but the response was too slow due to the inertia of moving mechanical components much larger than a probe tip.